For centuries, debating the nature of consciousness was the exclusive purview of philosophers. But if the recent torrent of books on the topic is any indication, a shift has taken place: Scientists are getting into the game.

Has the nature of consciousness finally shifted from a philosophical question to a scientific one that can be solved by doing experiments? The answer, as with any related to this topic, depends on whom you ask. But scientific interest in this slippery, age-old question seems to be gathering momentum. So far, however, although theories abound, hard data are sparse.

The discourse on consciousness has been hugely influenced by René Descartes, the French philosopher who in the mid-17th century declared that body and mind are made of different stuff entirely. It must be so, Descartes concluded, because the body exists in both time and space, whereas the mind has no spatial dimension.

Recent scientifically oriented accounts of consciousness generally reject Descartes's solution; most prefer to treat body and mind as different aspects of the same thing. In this view, consciousness emerges from the properties and organization of neurons in the brain. But how? And how can scientists, with their devotion to objective observation and measurement, gain access to the inherently private and subjective realm of consciousness?

Some insights have come from examining neurological patients whose injuries have altered their consciousness. Damage to certain evolutionarily ancient structures in the brainstem robs people of consciousness entirely, leaving them in a coma or a persistent vegetative state. Although these regions may be a master switch for consciousness, they are unlikely to be its sole source. Different aspects of consciousness are probably generated in different brain regions. Damage to visual areas of the cerebral cortex, for example, can produce strange deficits limited to visual awareness. One extensively studied patient, known as D.F., is unable to identify shapes or determine the orientation of a thin slot in a vertical disk. Yet when asked to pick up a card and slide it through the slot, she does so easily. At some level, D.F. must know the orientation of the slot to be able to do this, but she seems not to know she knows.

Cleverly designed experiments can produce similar dissociations of unconscious and conscious knowledge in people without neurological damage. And researchers hope that scanning the brains of subjects engaged in such tasks will reveal clues about the neural activity required for conscious awareness. Work with monkeys also may elucidate some aspects of consciousness, particularly visual awareness. One experimental approach is to present a monkey with an optical illusion that creates a "bistable percept," looking like one thing one moment and another the next. (The orientation-flipping Necker cube is a well-known example.) Monkeys can be trained to indicate which version they perceive. At the same time, researchers hunt for neurons that track the monkey's perception, in hopes that these neurons will lead them to the neural systems involved in conscious visual awareness and ultimately to an explanation of how a particular pattern of photons hitting the retina produces the experience of seeing, say, a rose.

Experiments under way at present generally address only pieces of the consciousness puzzle, and very few directly address the most enigmatic aspect of the conscious human mind: the sense of self. Yet the experimental work has begun, and if the results don't provide a blinding insight into how consciousness arises from tangles of neurons, they should at least refine the next round of questions.

Ultimately, scientists would like to understand not just the biological basis of consciousness but also why it exists. What selection pressure led to its development, and how many of our fellow creatures share it? Some researchers suspect that consciousness is not unique to humans, but of course much depends on how the term is defined. Biological markers for consciousness might help settle the matter and shed light on how consciousness develops early in life. Such markers could also inform medical decisions about loved ones who are in an unresponsive state.

Until fairly recently, tackling the subject of consciousness was a dubious career move for any scientist without tenure (and perhaps a Nobel Prize already in the bag). Fortunately, more young researchers are now joining the fray. The unanswered questions should keep them--and the printing presses--busy for many years to come.

"how can scientists, with their devotion to objective observation and measurement, gain access to the inherently private and subjective realm of consciousness?"

perception--consciousness--and "higher" levels of consciousness--provide life-long questions, and hopfully, grant funding for some lucky few that managed to join the fray, only to "keep(ing) printing presses busy for many years"! the other more mediocre ones are left to fill up holes of a big picture that has already emerged decades ago...

By using magnetic pulses to stimulate the brains of waking and sleeping volunteers, scientists may have gained an important insight into the age-old mystery of why consciousness fades as we nod off to sleep. In a report on page 2228, a research group at the University of Wisconsin (UW), Madison, concludes that as sleep sets in, communication between different parts of the cerebral cortex breaks down. Such communication is a likely prerequisite for consciousness, the team argues.

Some, but not all, neuroscientists find the team's evidence compelling. The research "definitely tells us something about sleep and may have important implications for understanding the neural correlates of consciousness," says Christof Koch, a cognitive neuroscientist at the California Institute of Technology in Pasadena.

Early neuroscientists assumed that consciousness wanes during sleep because the cerebral cortex simply shuts down. "In the last century, we had three Nobel Prize winners who thought that the cerebral cortex is completely inhibited during sleep," says Mircea Steriade, a neuroscientist who studies sleep at Laval University in Quebec, Canada. Electroencephalography (EEG) and other methods have since ruled out that explanation, showing that the electrical chatter and metabolism of neurons in the cortex continues unabated during sleep. That left neuroscientists puzzling over why consciousness fades when the brain is still active.

Giulio Tononi of UW has spent years developing a theory that the essence of consciousness is the integration of information. Communication between different regions of cortex might be one sign of this integration--and of consciousness, Tononi says. To test that idea, he and his team recorded electrical activity in the brains of six sleepy volunteers using high-density EEG. Before the subjects nodded off, the researchers stimulated a small patch of right frontal cortex with transcranial magnetic stimulation (TMS), a noninvasive method that uses magnetic pulses to induce an electrical current inside the head. The EEG record revealed how the neural activity triggered by TMS spread from the site of stimulation to other parts of the brain. The team repeated the experiment once the subjects had entered non-rapid eye movement (non-REM) sleep. Noise-canceling earphones ensured that subjects couldn't detect the sound of the TMS magnet.

When the subjects were awake, TMS elicited waves of neural activity that spread through neighboring areas of the right frontal and parietal cortex and to corresponding regions on the left side of the brain. During non-REM sleep, the same TMS stimulus only elicited neural activity at the site of stimulation.
Tononi says the findings suggest that different areas of cortex do indeed stop talking to each other during non-REM sleep--a stage of sleep in which people often report little or no conscious experience on waking. An important follow-up, he says, will be to repeat the experiments during late-night REM sleep, when people report consciouslike experiences in the form of dreams. "We would predict a pattern which is much more similar to wakefulness," he says.

Linking cortical connectivity to consciousness makes sense, says Rodolfo Llinas, a neuroscientist at New York University. A key feature of consciousness is the ability to integrate many aspects of an experience into a single perception--combining red petals, rosy scent, and prickly thorns into the perception of a rose, for example. "To make an object in your head, to make one single cognitive event, you have to bind the activity of many cortical areas," Llinas says.

But not everyone accepts Tononi's conclusions. The experiments are "very elegant and pretty," but their relevance to understanding consciousness is questionable, says Robert Stickgold, a neuroscientist who studies sleep at Harvard Medical School in Boston, Massachusetts. "There are many, many differences in brain chemistry and physiology ... between wakefulness, non-REM sleep, and REM sleep," including differences in neurotransmitter and hormone levels and patterns of neural activity, Stickgold says. The change in cortical communication is yet another such difference, he agrees, but there's no convincing evidence that it's the key to fading consciousness.